Does the Mac need Vulkan? It's not as if it was widely used. Games will still be coded with directX or console API. Vulkan will be as irrelevant as OpenGL for games.
So Apple is better off using Metal, an API they can control and which does the job.
Metal has more than 148 000 apps using it directly (mostly games I suppose) and 1.7M using it through higher level framework. Compared to the few dozens apps using Vulkan.